Entamoeba: The temperature ranges and optima for axenic growth of E. invadens 165 and E. histolytica were determined. Trophozoites were rapidly killed by exposure to temperatures 2-3 degrees above the minimum growth temperature, but killed very slowly at 0 degrees. Metabolically active E. histolytica cells were much more sensitive to killing by oxygen than cells at 0 degrees. Studies on the mechanism of attachment, motility and endocytosis have continued, with paritcular interest in the role of cysteine and ascorbic acid which cannot be replaced by other reducing agents or N2 in the defined composition "Motility Buffer" we formulated. Giardia lamblia: Studies on this common intestinal protozoan parasite have recently been initiated: attached and swimming trophozoites have been separated and their axenic growth and attachment characteristics are being determined. Our observation that L-cysteine is specifically required for cellular integrity, attachment and growth is being pursued. We have started studies on the cell surfaces of G. lamblia trophozoites and cysts using fluorescent and enzymatic probes.